1. Field of the Invention
The present invention relates to a solid-state imaging sensor, and more particularly, to a solid-state imaging sensor used in a ranging device.
2. Description of the Related Art
Ranging technology for auto-focus systems is known in digital still cameras and digital video cameras. In relation to ranging technology for auto-focus systems of this kind, Japanese Patent No. 04835136 proposes a solid-state imaging sensor which has a ranging function in a portion of the pixels of the solid-state imaging sensor, in such a manner that depth can be detected by a phase differential method. This phase differential method is a method in which two images which pass through different regions on the pupil in an imaging optical system (these images are herein after called “ranging images” below) are acquired, and the depth is detected by triangulation using a stereo image based on the amount of image shift between the ranging images. According to this method, unlike a conventional contrast method, there is no need to move the lens in order to measure the depth, and therefore a fast and highly accurate auto-focus system can be achieved.
However, depending on the imaging conditions, there may be a problem of decline in the ranging accuracy due to decline in the quality of the ranging images. In general, the exit pupil position of the imaging optical system varies depending on the zoom and focus conditions, and therefore the exit pupil position in the imaging optical system and the design pupil position of the imaging sensor do not necessarily coincide. If the design pupil position of the imaging sensor and the exit pupil position of the imaging optical system are different, then the amount of eccentricity of the pupil region through which the light beam received by each ranging pixel passes varies with the position of each ranging pixel in the imaging sensor. If the amount of eccentricity becomes large, then a differential occurs between the pupil transmittance distributions of the light beams which form the two ranging images. Furthermore, a differential also occurs between the pupil transmittance distributions of the light beams which form the two ranging images, due to vignetting.
If the exposure time is specified in accordance with a ranging image which has a relatively low pupil transmittance of the light beam forming the ranging image, then the ranging image having a relatively high pupil transmittance will readily become saturated. Conversely, if the exposure time is specified in accordance with a ranging image which has a relatively high pupil transmittance of the light beam forming the ranging image, then the ranging image having a relatively low pupil transmittance is liable to have an insufficient amount of light. From the above, since the quality of the ranging image declines, then the ranging accuracy also declines. When an object having a large contrast ratio is projected, the issue of decline in the quality of the ranging image is a particular problem.